The nutrients needed to make yolk for the egg come
from the hen's diet. Nutrients from food the hen eats
are absorbed from her digestive tract into her
bloodstream. The hen's liver takes these nutrients
from the bloodstream and turns them into yolk. Yolk
is then carried by the bloodstream from the liver to
the ovary. In the ovary the follicular cells
surrounding the oocyte take the yolk and other
nutrients from the bloodstream and pass them along to
the oocyte.

The immature oocytes and their surrounding
follicular cells lie deeply embedded within the ovary.
As the oocyte accumulates more and more yolk, it
becomes too big to fit inside the ovary.
Consequently, the oocyte and the ovarian follicle it
is nested in is continually pushed toward the outer
edge of the ovary, until it is connected to the ovary
by only a stalk. When the oocyte has accumulated
enough yolk to grow a chick, the oocyte ruptures from
its follicle. This process is called ovulation.

The free oocyte is guided by the infundibulum into
the mouth of the hen's left oviduct. (In the female
chicken only the left ovary and oviduct develop.)

In the Oviduct

Shortly after release from the ovary, the egg's
nucleus undergoes meiosis to prepare it for
fertilization. Only one of the cells produced during
meiosis goes on to become a mature oocyte, while the
rest fade away. The mature oocyte now contains a
haploid number of chromosomes and is ready to be
fertilized by a sperm. Inside the lower portion of
the oviduct are special crypts where sperm from a
rooster can be stored and remain viable for up to
three weeks. Ovulation causes the walls of the
oviduct to contract and cilia lining the walls to
move, causing an upward current in the oviduct. If
sperm are present in the crypts of the lower oviduct,
they will be swept toward the mature oocyte as it
enters the oviduct and fertilization takes place.

Whether the oocyte is fertilized or not, it will
continue down the oviduct to be covered by layers of
albumen (egg white) and internal supporting
structures. The portion of the oviduct that secrets
albumen is called the magnus. The oocyte and its
surrounding layers can now be called an egg. In its
upper portion the oviduct adds an outer vitelline
membrane to the egg. Because the inside walls of the
oviduct are arranged in downward spiralling folds, the
egg rotates as it travels down the oviduct. As the
egg twists, protein fibers extending from the new
vitelline membrane capture the thick and thin albumens
secreted along the oviduct. This is how the chalazae
and layers of albumen are formed. The shell membranes
are added next. The shell gland, located toward the
end of the oviduct in the uterus, adds the shell. It
takes about 24 hours for the egg to travel down the
oviduct. Hens tend to lay eggs during the middle of
the day; if the egg is not complete until later in the
day, it will remain at the end of the oviduct until
the next day.